1. Field of the Invention
The present invention relates to a vibration gyro used for detecting an angular velocity.
2. Description of The Related Art
Vibration gyros have been used for detecting shaking, for example, in shaking prevention mechanisms of household video cameras or position detection mechanisms of car navigation systems, or for detecting a rotation speed of a vehicle body.
According to laws of physics, when viewed from a coordination system rotating at an angular velocity of Ω, a Coriolis force Fc proportional to the relative velocity V of a moving physical body acts upon the physical body in the direction perpendicular to that of the velocity. The magnitude and direction of this force are represented by the following equation:Fc=2 mV×Ω  (1)Here, m is the mass of the physical body upon which the Coriolis force acts.
A vibration gyro is an angular velocity sensor in which a resonator is caused to vibrate and a rotation is detected by determining a Coriolis force acting in the direction orthogonal to the vibration direction of the resonator due to the rotation. A variety of types such as a tuning piece type and a tuning fork type have been suggested for the resonator used in the vibration gyro.
Forces relating to two vibrations act upon the resonator of a vibration gyro: a drive vibration causing the resonator to vibrate and a detection vibration induced by a Coriolis force acting upon the resonator due to rotation.
It is preferred that neither the drive vibration nor the detection vibration of the vibration gyro be affected by the support section that supports the resonator. Furthermore, it is also preferred that the vibration gyro have a small drift and good S/N.
A vibration gyro using a resonator having three legs 5, as shown in FIG. 13 has been suggested as a vibration gyro with a small drift, good S/N and no effect from the support section (Japanese Patent Application Laid-open No. 2003-156337).
This resonator has a support section 7, a base section 3, and three legs 5. The base section 3 and the three legs 5 have substantially the same thickness. The three legs 5 have the same length and are arranged parallel to each other. The width W1 of one leg of the two side legs is substantially equal to the width W2 of the central leg and those two legs are used as drive legs 15. The width W3 of the other side leg is about ⅗ the widths W1, W2 of the two aforementioned legs, and this other leg is a detection leg 17.
The detection vibration of the resonator is performed by inducing vibrations in the drive legs 15. The drive vibration is performed by causing the two drive legs 15 to vibrate in the direction in which those two legs are arranged. Thus, both drive legs 15 are caused to vibrate in a plane formed by the two drive legs 15.
The two drive legs 15 perform flexural vibrations and repeatedly come close to each other and withdraw from one another. The two drive legs 15 perform the vibrations of the so-called tuning fork type. The two drive legs 15 have substantially equal natural vibration frequencies in the above-described in-plane vibrations. Furthermore, because the two drive legs 15 have substantially same thicknesses, lengths, and widths, the vibrations of the drive legs 15 are balanced. As a result, practically no vibration leaks to the support section 7.
Furthermore, because the width W3 of the detection leg 17 is ⅗ the widths W1, W2 of the drive legs 15, the natural vibration frequency of the detection leg is significantly different from the natural vibration frequency of the drive legs 15. For this reason, the two drive legs 15 vibrate so that the drive legs 15 are balanced, whereas the detection leg 17 is in an almost perfectly stationary state.
When this resonator is rotated, as described hereinabove, a Coriolis force is generated in the direction perpendicular to the vibration direction of the drive legs 15. Thus, a Coriolis force is generated in the direction perpendicular to the plane formed by the two drive legs 15. Under the effect of this Coriolis force, the drive legs 15 start vibrating in the direction perpendicular to this plane. In other words, the two drive legs 15 start vibrating in the out-of-plane direction. The detection leg 17 stands still when no rotation is provided. If a Coriolis force is generated in the two drive legs 15, the detection leg 17 starts vibrating in the out-of-plane direction to balance this Coriolis force. This vibration of the detection leg 17 is called “detection vibration”.
The vibration amplitude of the detection leg 17 is proportional to the Coriolis force. Equation (1) shows that the Coriolis force is proportional to the angular velocity. Therefore, if the vibration of the detection leg 17 is detected, the angular velocity can be found. Because the width W3 of the detection leg 17 is ⅗ the widths W1, W2 of the drive legs 15, the movement of the two drive legs 15 and the detection leg 17 is balanced even in the out-of-plane vibrations. Therefore, the vibration leak to the support section 7 is practically zero.
In the vibration gyro using such resonator, both the drive vibration and detection vibration are accompanied by practically no vibration leak to the support section 7. For this reason, the Q of drive vibration and detection vibration is high and a large detection signal S can be obtained. Furthermore, when no rotation is provided to the resonator (stationary state), the detection leg 17 stands still, and this leg starts out-of-plane vibrations for the first time only when the rotation is provided. For this reason, noise N is low. As a result, high S/N and low drift are realized.
An increase in accuracy is required for a vibration gyro using such a resonator. The problem is that a higher S/N is necessary to obtain a high accuracy. Vibration leak has to be further reduced to obtain a higher S/N.
Accordingly, it is an object of the present invention to provide a vibration gyro capable of realizing high S/N, without vibration leak.